Synthesis, characterization and X-ray structure of [Pd(SO4)(dppp)] · H2O, a catalyst for the CO-ethene copolymerization [dppp = 1,3-bis(diphenylphosphino)propane]

The title complex has been synthesized by first reacting dppp with Pd(AcO)₂ in acetone and then with NaHSO₄ in water. It has been characterized by IR, NMR and X-ray diffraction studies. The ³¹P NMR spectrum in DMSO shows a singlet at 16.62 ppm indicating that the two P atoms are equivalent and that the sulfate anion is weakly coordinating. The X-ray structure shows that the Pd atom is surrounded in an almost regular square planar environment by the two P atoms and by two O atoms of the sulfate anion and that the neutral complex is accompanied by a water molecule of crystallization. The Pd-P distances (2.217(1) and 2.233(1)) and the P-Pd-P angle (90.78(3)°) are close to those found in other complexes where the chelating diphosphine is the same. Also the Pd-O distances and the O-Pd-O bond angle are comparable to those of other relevant chelating ligands.In MeOH, the title complex, in combination with H₂SO₄, catalyses the CO-ethene copolymerization. The productivity reaches a maximum upon increasing the H₂SO₄/Pd ratio up to ca. 470 (7650 g of polyketone/g Pd h at 90 °C and 45 atm, CO/ethene 1/1). The viscosity of the polyketone passes through a maximum of 0.95 dL/g in m-cresol when the above ratio is ca. 100. It has been proposed that acid promotes the copolymerization process by destabilizing the β- and γ-chelates intermediates involved in chain growing process, thus favoring the insertion of the monomers. At relatively high acid concentration the lowering of productivity and viscosity suggests that the sulfate anion competes with the monomers for the coordination to the metal center.In H₂O-CH₃COOH as a solvent the productivity strongly depends on the H₂O/CH₃COOH ratio, as it passes through a maximum of 12 000 g polymer/g Pd h in the presence of ca. 60% of H₂O. The productivity is significantly lower than that found when the acetate and chloride analogues are used (27 000 g polyketone/g Pd · h). Thus, it is likely that the sulfate anion assists significantly the copolymerization process even though the concentration of CH₃COOH/CH₃COO⁻ is much preponderant.